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From: Andy Lutomirski <luto@amacapital.net>
Date: Wed, 31 Jan 2024 11:24:48 -0800
Message-ID: <CALCETrXTHsyiR6Bav7bXCCHny0Z2Bn90fTUL9__KTftESQ9=7w@mail.gmail.com>
Subject: Re: [RFC PATCH] pidfd: implement PIDFD_THREAD flag for pidfd_open()
To: Oleg Nesterov <oleg@redhat.com>
Cc: Tycho Andersen <tycho@tycho.pizza>, Christian Brauner <brauner@kernel.org>, linux-kernel@vger.kernel.org, 
	linux-api@vger.kernel.org, Tycho Andersen <tandersen@netflix.com>, 
	"Eric W. Biederman" <ebiederm@xmission.com>
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> On 01/31, Oleg Nesterov wrote:
> >
> > On 01/31, Andy Lutomirski wrote:
> > Please note
> >
> >       /* TODO: respect PIDFD_THREAD */
> >
> > this patch adds into pidfd_send_signal().
> >
> > See also this part of discussion
> >
> >       > > +   /* TODO: respect PIDFD_THREAD */
> >       >
> >       > So I've been thinking about this at the end of last week. Do we=
 need to
> >       > give userspace a way to send a thread-group wide signal even wh=
en a
> >       > PIDFD_THREAD pidfd is passed? Or should we just not worry about=
 this
> >       > right now and wait until someone needs this?
> >
> >       I don't know. I am fine either way, but I think this needs a sepa=
rate
> >       patch and another discussion in any case. Anyway should be trivia=
l,
> >       pidfd_send_signal() has the "flags" argument.
> >
> > with Christian in https://lore.kernel.org/all/20240130112126.GA26108@re=
dhat.com/

I missed that.  Whoops.

On Wed, Jan 31, 2024 at 11:15=E2=80=AFAM Oleg Nesterov <oleg@redhat.com> wr=
ote:
>
> Forgot to mention...
>
> And I agree that pidfd_send_signal(flags =3D> PGID/SID) can make
> some sense too.
>
> But this a) doesn't depend on PIDFD_THREAD, and b) needs another
> patch/discussion.
>
> But again, I am not sure I understood you correctly.
>

Hmm.

When one works with regular (non-fd) pids / pgids etc, one specifies
the signal domain at the time that one sends the signal.  I don't know
what pidfds should do.  It seems a bit inefficient for anything that
wants a pidfd and might send a signal in a different mode in the
future to have to hold on to multiple pidfds, so it probably should be
a pidfd_send_signal flag.

Which leaves the question of what the default should be.  Should
pidfd_send_signal with flags =3D 0 on a PIDFD_THREAD signal the process
or the thread?  I guess there are two reasonable solutions:

1. flags =3D 0 always means process.  And maybe there's a special flag
to send a signal that matches the pidfd type, or maybe not.

2. flags =3D 0 does what the pidfd seems to imply, and a new
PIDFD_SIGNAL_PID flag overrides it to signal the whole PID even if the
pidfd is PIDFD_THREAD.

Do any of you have actual use cases in mind where one choice is
clearly better than the other choice?

--Andy